Golf ball

Abstract: Disclosed is a golf ball having excellent (soft) hit feeling and excellent durability, which is suitable for using on the driving range. The golf ball has a two-layer structure comprising a core and a cover for coating the core.The compressive strain of the core is 2.8 to 3.8 mm. The hardness distribution of the core (measured by a JIS-C type hardness tester) is adjusted to 65 to 79 at the center, 70 to 80 at the location which is 5 mm away from the center to the surface, 73 to 80 at the location which is 10 mm away from the center to the surface, 75 to 82 at the location which is 15 mm away from the center to the surface and 70 to 85 at the surface, and a difference in hardness between adjacent locations of measurement is within 5. The cover comprises an ionomer resin as the main material the stiffness of the cover is 1400 to 3000 kg/cm². The ball compression is 70 to 100 (PGA system).

Description: The present invention relates to a golf ball having a two-layer structure comprising a core and a cover for covering the core (two-piece golf ball), which is particularly suitable for using in driving ranges. Heretofore, a one-piece golf ball has exclusively been used as golf balls used in driving ranges in consideration of durability. However, problems associated therewith include that the one-piece golf ball is extremely inferior in flight performances and hit feeling in comparison with golf ball used for rounds of golf. Of course, a golfer would prefer to use the golf ball as used for rounds of golf at driving ranges. A thread wound golf ball (a golf ball obtained by winding a thread rubber on a center and covering the resulting thread rubber layer with a cover) is extremely superior in hit feeling, but is however extremely inferior in durability, and furthermore is expensive. Therefore, the thread wound golf ball is not suitable for a golf ball for use at driving ranges. Further, a golf ball which is used for a round of golf having a two-piece structure in which a solid core is covered with a cover is too rigid for repetitive hitting and, therefore, normal practice can not be conducted. Therefore, this golf ball is not also suitable for use at driving ranges. Therefore, a trial of softening the core of a golf ball to cushion the shock upon hitting has been conducted. However, even when the core is softened, the durability of the ball becomes inferior because of a large difference in hardness between the cover and the core. Therefore, this ball is not also suitable for practical use. As described above, a conventional golf ball for practice at driving ranges is inferior in flight performances and hit feeling in comparison with a golf ball for a round of golf whereas, the golf ball for the round of golf is inferior in durability and is too rigid for repetitive hitting. Therefore, they are not suitable as golf balls to be used at driving ranges. JP-A-60090575 discloses a two-piece golf ball according to the preamble of claim 1, which golf ball has a hardness distribution which is within the ranges described in the characterising portion of claim 1. EP-A-0589647 also discloses a two-piece golf ball and is comprised in the state of the art by virtue of Article 54(3) EPC. The main object of the present invention is to provide a golf ball having excellent hit feeling and durability, which is particularly suitable for using as a golf ball for use at driving ranges. These objects as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description. The present invention provides a golf ball having a two-layer structure comprising a core and a cover for covering the core; the compressive strain of said core being 2.8 to 3.8 mm, the hardness distribution of said core (measured by a JIS-C type hardness tester) being adjusted to from 65 to 79 at the center, 70 to 80 at a location which is 5 mm away from the center to the surface, 73 to 80 at a location which is 10 mm away from the center to the surface, 75 to 82 at a location which is 15 mm away from the center to the surface and 70 to 85 at the surface, wherein the difference in hardness between adjacent locations of measurement is within 5; said cover comprising an ionomer resin as a main material, the stiffness of said cover being 1400 to 3000 kg/cm 2 ; and the ball compression being 70 to 100 (PGA system). According to the present invention, a golf ball having soft hit feeling and excellent durability which is particularly suitable for using at driving ranges is obtained by using an ionomer resin as a main material of a cover and decreasing the stiffness of the cover to from 1400 to 3000 kg/cm 2 smaller than that of the cover used for a normal golf ball for a round of golf to soften the cover; increasing the compressive strain of the core to from 2.8 to 3.8 mm larger than that of a normal core to soften the core so as to adapt to the soft cover; and limiting the hardness distribution of the core to a specific one as described above and decreasing the ball compression to from 70 to 100 (PGA) to soften the entire golf ball in comparison with a normal golf ball for a round of golf. In the present invention, the compressive strain of the core is from 2.8 to 3.8 mm. When the compressive strain of the core is smaller than 2.8 mm, the cores becomes too rigid, which results in inferior hit feeling. When the compressive strain of the core is larger than 3.8 mm, the cores becomes too soft, which results in inferior durability. In the present invention, the hardness distribution of the core (measured by a JIS-C type hardness tester) is adjusted to from 65 to 79 at the center, 70 to 80 at the location which is 5 mm away from the center to the surface, 73 to 80 at the location which is 10 mm away from the center to the surface, 75 to 82 at the location which is 15 mm away from the center to the surface and 70 to 85 at the surface, wherein the difference in hardness between adjacent locations of measurement is within 5. By adjusting the hardness distribution of the core as described above, the compressive strain of the core can be moderately maintained, which results in good hit feeling. When the hardness measured at each location of measurement is higher than that in the above hardness distribution, the compressive strain of the core becomes small, which results in inferior hit feeling. When the hardness measured at each location of measurement is lower than that in the above hardness distribution, the compressive strain of the core becomes large, which results in inferior hit feeling and durability. By adjusting the difference in hardness between adjacent locations of measurement within 5, a golf ball having excellent durability and hit feeling can be obtained. The hardness of the interior of the core can be measured by cutting the core into hemispherical pieces, followed by measuring the hardness at the above specific location of measurement. The center, a location which is 5 mm away from the center to the surface, a location which is 10 mm away from the center to the surface,a location which is 15 mm away from the center to the surface and surface (which are normally employed as the location of measurement in case of determination of the hardness distribution of the core) are selected for determination of the hardness distribution of the core, because the hardness distribution can not be given unless the location of measurement is specified. In the golf ball of the present invention, the core consists of a single layer and the hardness thereof varies continuously. On the other hand, in a core having a multi-layer structure, the hardness varies discontinuously with the layers. The core having different hardness distribution in the single layer structure as described above can be obtained by selecting a vulcanizing agent and vulcanizing conditions. In the present invention, the hardness is defined as that measured by a JIS-C type hardness tester. The JIS-C type hardness tester is a spring type hardness tester (C type) according to JIS K 6301 (procedure of physical test of vulcanized rubber). The stiffness of the cover of the golf ball of the present invention is from 1400 to 3000 kg/cm 2 . When the stiffness is smaller than 1400 kg/cm 2 , scratches are liable to be formed on the surface of the cover. When the stiffness is larger than 3000 kg/cm 2 , the durability of the ball becomes inferior. In the present invention, the ball compression is from 70 to 100 (PGA system), preferably 70 to 95 (PGA system). When the ball compression is smaller than 70 (PGA system), the durability of the golf ball deteriorates. When the ball compression is larger than 100 (PGA system), the hit feeling becomes rigid (not soft). The core having the above characteristics may comprise a vulcanized product of a rubber composition. As the rubber component of the rubber composition, for example, butadiene rubber having a cis-1,4-structure (base rubber) is suitable. The total rubber component may include other rubbers such as natural rubber, styrene-butadiene rubber, isoprene rubber, chloroprene rubber, butyl rubber, ethylene-propylene rubber, ethylene-propylene-diene rubber, acrylonitrile, etc.which are blended with the above butadiene rubber in an amount of not more than 40 parts by weight based on 100 part by weight of the butadiene rubber component. As the vulcanizing agent, there can be used those which are normally used as vulcanizing agents, for example, metal salts of α, β-ethylenicallly unsaturated carboxylic acids obtained by reacting α, β-ethylenicallly unsaturated carboxylic acids such as acrylic acid and methacrylic acid with metal oxides such as zinc oxide in the preparation of the rubber composition, metal salts (normal salt or basic salt) of α, β-ethylenicallly unsaturated carboxylic acids such as zinc acrylate and zinc methacrylate, polyfunctional monomers, N,N-phenylbismaleimide, sulfur and the like. Among them, metal salts (particularly, zinc salt) of α, β-ethylenicallly unsaturated carboxylic acids are particularly preferred. The amount of the vulcanizing agent is preferably 20 to 40 parts by weight (in case of metal salts of α, β-ethylenicallly unsaturated carboxylic acids) based on 100 parts by weight of the rubber component. When α, β-ethylenicallly unsaturated carboxylic acids are reacted with metal oxides in the preparation of the rubber composition, the amount of α, β-ethylenicallly unsaturated carboxylic acids is preferably 15 to 30 parts by weight and the amount of metal oxides such as zinc oxide is preferably 15 to 35 parts by weight, based on 100 parts by weight of the rubber component. As the filler, for example, there can be used at least one sort of inorganic powder such as barium sulfate, calcium carbonate, clay, zinc oxide and the like. The amount of the filler is preferably 5 to 50 parts by weight based on 100 parts by weight of the rubber component. A suitable amount of a softening agent and liquid rubber may be formulated for the purpose of improving workability or adjusting hardness. Further, a suitable amount of an antioxidant may be formulated for the purpose of preventing ageing. As the vulcanization accelerator, for example, there can be used organic peroxides such as dicumyl peroxide, 1,1-bis(t-butyl peroxy)3,3,5-trimethylcyclohexane and the like. The amount of the vulcanization accelerator is preferably 0.1 to 5 parts by weight, particularly 0.3 to 3 parts by weight, based on 100 parts by weight of the rubber component. In the preparation of the core, crosslinkage due to sulfur is not necessarily required for vulcanization of the rubber composition. A more appropriate term to use may be "crosslinking" rather than "vulcanization". In the present specifications, however, we use the term "vulcanization" in accordance with precedent. In the preparation of the core, the above formulation materials are mixed using a roll, kneader, Banbury, etc. and the mixture is vulcanized at 145 to 200°C, preferably 150 to 175°C under pressure for 10 to 40 minutes using a mold. In order to improve the adhesion of the resulting core to the cover, an adhesive may be applied on the surface thereof or the surface may be roughened. The cover comprises an ionomer resin as the main material and the stiffness is adjusted to 1400 to 3000 kg/cm 2 by blending at least one ionomer resin. In addition to the ionomer resin, titanium oxide (TiO 2 ), light stabilizers, colorants, antioxidants and the like may be formulated, if necessary. Further, a part of the ionomer resin may be substituted with other polymers such as polyethylene, polyamide and the like unless properties of the ionomer resin (e.g. excellent cut resistance, etc.) are not damaged. As the cover of a normal golf ball such as a golf ball for a round of golf, ionomer resins such as Hi-milane #1605, Hi-milane #1705, Hi-milane #1706 (trade name, manufactured by Mitsui Du Pont Polychemical Co.) are sometimes used in combination. However, it is sometimes difficult to adjust the stiffness to the range of 1400 to 3000 kg/cm 2 by only using these ionomer resins. In the present invention, it is preferred to adjust the stiffness to the range of 1400 to 3000 kg/cm 2 using an ionomer resin having low stiffness such as Hi-milane #1855 (trade name, manufactured by Mitsui Du Pont Polychemical Co., stiffness of 917 k/cm 2 ). The cover having the above stiffness is soft in comparison with the cover used for a golf ball for a round of golf. By using the soft cover, the hit feeling becomes soft and the cover adapts to the softened core to prevent deterioration of the durability due to mismatching of the cover and core, which results in excellent durability. When using the soft cover, the durability is deteriorated due to mismatching of the cover and core if the core is soft. In the present invention, since the core is also softened, the durability is not deteriorated. In the present invention, the soft hit feeling is considered to be good because it is suitable for hitting a lot of golf balls. Hi-milane (trade name) manufactured by Mitsui Du Pont Polychemical Co. was given above as an example of an ionomer resin. However, the ionomer resin is not limited to a specific one, for example, there can also be used those which are commercially available under the trade name of ESCOR and IOTEK manufactured by Exxon Chemical Co. In the blending of the above ionomer resins, those obtained by neutralizing with a sodium ion may be blended with those obtained by neutralizing with a zinc ion. It is however preferred that those obtained by neutralizing with a zinc ion are blended together. When the core is coated with the above cover, an injection molding method is normally used owever, it is not limited to a specific method, for example, coating may be conducted by a molding method after preparing a half-shell. The thickness of the cover is not specifically limited, and it is normally 1.4 to 2.7 mm. In case of cover molding, dimples may be formed, if necessary. Further, if necessary, a paint or marking may be applied after cover molding. EXAMPLES The following non-limiting Examples and Comparative Examples further illustrate the present invention in detail: Examples 1 to 4 and Comparative Examples 1 to 3 The formulation components shown in Tables 1 and 2 were kneaded to prepare rubber compositions for the cores of Examples 1 to 4 and Comparative Examples 1 to 3. After forming into a sheet, the rubber sheet was placed in a mold and subjected to a vulcanization molding in a press under conditions shown in Tables 1 and 2 to prepare a core of 34.8 mm in diameter. The amount of each component in Tables 1 and 2 is "parts by weight". The weight, the compressive strain and the hardness distribution of the core thus obtained were measured. The results are shown in Tables 1 and 2. The formulation, vulcanizing conditions and physical properties of the cores of Examples 1 to 4 are shown in Table 1. Those for Comparative Examples 1 to 3 are shown in Table 2. Further, details of formulation components in Tables 1 and 2 are described following Table 2 The measuring method of the compressive strain and the hardness distribution of the core is as follows. Compressive strain: An initial load (10 kg) is applied on the core, and then a final load (130 kg) is applied. The amount of deformation (mm) formed between initial loading and final loading is measured as the compressive strain. The larger the value is, the softer the core. Hardness distribution: The hardness was measured at the center of the core, locations which are respectively 5 mm, 10 mm and 15 mm away from the center to the surface of the core and the surface of the core, using a JIS-C type hardness tester. The larger the value is, the more rigid the core. The hardness of the core is measured after cutting the core into hemispherical pieces. Example 1 Example 2 Example 3 Example 4 Butadiene rubber 100 100 100 100 Zinc acrylate 0 0 0 30 Zinc oxide 30.5 30.5 30.5 20 Antioxidant 0.2 0.2 0.2 0.25 Methacrylic acid 18 18 18 0 Dicumyl peroxide 1.4 1.8 1.8 2.0 Vulcanizing condition (°C x minutes) 155 x 30 155 x 25 155 x 25 160 x 20 Physical properties of core Weight (g) 35.2 35.2 35.2 35.2 Compressive strain (mm) 3.5 3.1 3.1 3.2 Hardness distribution (JIS-C) Center 68 74 74 67 Location which is 5 mm away from the center 70 76 76 70 Location which is 10 mm away from the center 73 77 77 73 Location which is 15 mm away from the center 75 79 79 78 Surface 73.5 76.5 76.5 83 Comparative Example 1 Comparative Example 2 Comparative Example 3 Butadiene rubber 100 100 100 Zinc acrylate 0 0 0 Zinc oxide 30.5 28.7 29.2 Antioxidant 0.2 0.25 0.2 Methacrylic acid 18 24 21 Dicumyl peroxide 1.4 1.5 1.5 Vulcanizing condition (°C x minutes) 155 x 35 170 x 22 175 x 32 Physical properties of core Weight (g) 35.2 35.1 35.2 Compressive strain (mm) 3.5 2.4 3.1 Hardness distribution (JIS-C) Center 68 73 64 Location which is 5 mm away from the center 70 75 67 Location which is 10 mm away from the center 71 80 71 Location which is 15 mm away from the center 74 84 73 Surface 73.5 86 80 Then, a cover composition was prepared according to the formulation shown in Table 3, and the stiffness thereof was measured. The results are shown in Table 3. The amount of the formulation component is "parts by weight" and the measuring method of the stiffness is as follows. Stiffness: A cover composition is subjected to a press molding to prepare a plate specimen, which is allowed to stand at a temperature of 23°C and a relative humidity of 50% for two weeks and the stiffness is measured by a stiffness meter manufactured by Toyo Seiki Co., Ltd. Formulation of cover A B C Himilane #1855 15 50 0 Himilane #1705 25 20 10 Himilane #1706 60 30 90 Titanium oxide (TiO 2 ) 1.0 1.0 1.0 Stiffness (kg/cm 2 ) 2400 1600 3200 As shown in Table 3, formulations A and B of the cover belong to the present invention because the stiffness thereof is in the range of 1400 to 3000 kg/cm 2 . However, the formulation C is not included in the present invention because the stiffness thereof exceeds 3000 kg/cm 2 . Then, a core was coated with a cover according to the manner as shown in Tables 4 and 5 to prepare a golf ball of 42.7 in diameter. The coating of the cover on the core was conducted at a temperature of 230°C by an injection molding method. As to the resulting golf ball, the weight, the compression, the durability and the hit feeling were determined. The results are shown in Tables 4 and 5. The cover formulation, the weight, the compression, the durability and the hit feeling of the resulting golf ball as to Examples 1 to 4 are shown in Table 4. Those as to Comparative Examples 1 to 4 are shown in Table 5. The measuring method of the compression, the durability and the hit feeling shown in Tables 4 and 5 is as follows. Compression (ball compression): It is conducted according to PGA system. The larger the value is, the more rigid the golf ball. Durability: A gold ball was struck against a metal plate at a speed of 45 m/second by an air gun, and the number of times until the golf ball was broken was measured. The resulting value was indicated as an index in case of the value of the golf ball of Example 3 being 100. The larger the value is, the better the durability. Hit feeling: A total of one hundred golfers of two professional golfers and ninety-eight amateur golfers actually hit the golf ball on the driving range and the hit feeling was evaluated in the following criteria: good (soft), ordinary and inferior (rigid). Example 1 Example 2 Example 3 Example 4 Formulation of cover A A B A Physical properties of ball Weight (g) 45.3 45.4 45.4 45.5 Compression 73 90 86 87 Durability 98 99 100 97 Hit feeling Good (soft) 95 88 91 87 Ordinary 5 11 9 13 Inferior (rigid) 0 1 0 0 Comparative Example 1 Comparative Example 2 Comparative Example 3 Formulation of cover C A C Physical properties of ball Weight (g) 45.3 45.2 45.2 Compression 79 106 98 Durability 51 97 61 Hit feeling Good (soft) 41 0 0 Ordinary 59 8 19 Inferior (rigid) 0 92 81 As is shown in Table 4, the gold balls of Examples 1 to 4 of the present invention were superior in hit feeling and durability. On the other hand, as shown in Table 5, the golf balls of Comparative Examples 1 to 3 are inferior in hit feeling and/or durability. That is, the golf ball of Comparative Example 1 having high stiffness of the cover is inferior in durability, and the golf ball of Comparative Example 2 having low compressive strain of the core is inferior in hit feeling. The hardness of the core measured at each location of measurement is low in comparison with the present invention. The golf ball of Comparative Example 3 having high stiffness of the cover is inferior in both hit feeling and durability. As described above, according to the present invention, there can be provided a golf ball having excellent hit feeling and durability which is suitable as a golf ball used in the driving range by softening the cover and further softening the core so as to adapt to the soft cover.

Claims: A golf ball having a two-layer structure comprising a core and a cover for coating the core, said cover comprising an ionomer resin as a main material, characterized in that the compressive strain of the core is in the range of from 2.8 to 3.8 mm, the hardness distribution of the core (measured by a JIS-C type hardness tester) is in the range of from 65 to 79 at the center, 70 to 80 at a location which is 5 mm away from the center to the surface, 73 to 80 at a location which is 10 mm away from the center to the surface, 75 to 82 at a location which is 15 mm away from the center to the surface and 70 to 85 at the surface with a difference in hardness between adjacent locations of measurement being within 5, and in that the stiffness of the cover is in the range of from 1400 to 3000 kg/cm 2 and the ball compression is in the range of from 70 to 100 (PGA system). A golf ball as claimed in claim 1 wherein the core comprises a vulcanised product of a rubber composition. A golf ball as claimed in claim 1 or claim 2 wherein the rubber composition is butadiene rubber having a Cis-1,4-structure. A golf ball as claimed in claim 2 or claim 3 wherein the rubber composition further comprises one or more other rubbers selected from natural rubber, styrene-butadiene rubber, isoprene rubber, chloroprene rubber, butyl rubber, ethylene-propylene rubber, ethylene-propylene diene rubber or acrylonitrile. A golf ball as claimed in any one of the preceding claims wherein the core comprises a filler, softening agent or antioxidant or two or more thereof. A golf ball as claimed in any one of the preceding claims wherein the cover further comprises titanium oxide, light stabilizers, colorants or antioxidants or two or more thereof. A golf ball as claimed in any one of the preceding claims wherein a part of the ionomer resin is substituted with other polymers. A golf ball as claimed in claim 7 wherein the other polymers include polyethylene or polyamide or a mixture thereof.

Inventors: HIRAOKA HIDENORI, HIRAOKA, HIDENORI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1994
Main classification: A63B37
IPC codes: A63B37